We recently received a letter from a reader asking us which acids might be used to clean a bronze propeller. Although we’ve found it fairly easy to remove even the most aggressive fouling with scrapers, wet-dry sandpaper, and Scotchbrite pads, we know that some props are harder to clean than others. Folding propellers in particular have nooks and crannies that are hard to access. And barnacle shells are tenacious! So what is the story with acids?
Just a few barnacles in the wrong place can prevent folding. Lack of grease can cause sticking. A film of growth and scale just 1 mm thick can reduce efficiency 12 to 15 percent. Annual, or even more frequent maintenance, is a must.
Warning! To prevent accidental starting, remove the ignition key and secure, and turn off the power at the main switch before working on any propeller. This goes triple if the boat is in the water.
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Maintenance requirements vary too widely for us to make general recommendations. Some folding or feathering props require full annual disassembly, cleaning, and greasing, while others suggest only cleaning. Inspect the prop multiple times each season. Try to remove barnacles as they form, while they are soft and before they fully anchor. Replace anodes before they are 50 percent expended, which can vary from 1-3 years. Blades are balanced, so mark them with a sharpie before disassembly. Here are abbreviated recommendations from the makers of some common folding or feathering propellers.
• Maxprop. Annual greasing with special procedure to pack the gears full.
• Varifold. Full annual disassembly and cleaning. Bumper pads will need to be replaced.
• Gori. Full annual disassembly and greasing. They recommend Barnacle Buster used at 5-20% acid concentration.
• Flexofold. Clean in place. Grease on the pins helps but is not required.
Based on our tests of descaling solutions’ effect on brass and copper (“Descaling Solutions for Boats,” see PS August 2017), and taking into consideration Gori’s advice, we’d stick with Barnacle Buster. Use the standard product as-is. If using Barnacle Buster Concentrate, dilute it as needed to a concentration somewhere between 20:1 to 5:1, starting with a low concentration. Never add water to acid! Only add acid to water. Wear goggles and gloves. Only muriatic acid (hydrochloric acid) did measurable harm in our tests, and only after considerable soaking.
If you use acid to descale, you will wash out the grease, so regrease.
Also clean the anode if you’re not replacing it. Acid will help remove the oxidized crust. Hit the anode with a wire brush to reactivate it.
